1. Field of the Invention
The present invention relates generally to an electrochemical cell. More particularly, the present invention relates to an electrochemical cell having a stack holder that keeps the electrodes in proper electrochemical alignment and close proximity with respect to each other, even as their dimensions change during cell discharge.
2. Description of Related Art
A typical electrochemical cell used to power an implantable medical device comprises a casing housing an anode and a cathode. The anode and the cathode are physically segregated from each other, typically by enclosing at least one of them within an envelope or bag of insulative separator material. The separator is typically provided as a thin porous sheet material that is saturated with electrolyte and allows the transport of ions in the electrolyte there through. The anode and the cathode are generally formed as one or more respective plates of anode active material and cathode active material. The plates are then aligned face-to-face and spaced apart from each other by the separator material, to form an electrode assembly or electrode stack (a.k.a., cell stack) within the cell casing. In order to maximize discharge efficiency and stabilize the location of the electrodes within the casing, it is preferable that the electrode assembly be tightly fitted within the casing's walls while occupying as much internal volume as possible. It is understood that electrochemical cells perform most efficiently when the anode and the cathode plates are positioned in close physical proximity to each other. Close physical proximity minimizes the path length that current carrying ions must travel. Ultimately, the close physical proximity minimizes the electrical impedance of the cell as measured at the cell terminals.
During discharge of lithium anode-type cells, the thicknesses of the cathode and anode plates change. In lithium anode systems, the thicknesses of the cathode plates increase while those of the anode decrease, but the total thickness of the electrode assembly decreases continuously throughout discharge. This occurs because the rate of cathode thickness increase due to lithium intercalation is smaller than the rate of lithium consumption at the anode. As the overall electrode assembly thickness decreases, gaps can form between the anode plates and the cathode plates. The gaps may eventually be sufficient to allow the electrode assembly to move within the casing. That outcome is undesirable. As the electrodes form gaps, they may no longer be in close physical proximity to each other. As the cell discharges, this may result in an increase in electrical impedance along with increased cell resistance between the cathode and the anode, thereby causing lower pulse voltages (cell terminal voltage under conditions after an intermittent high current pulse load), faster cell polarization, greater voltage fluctuations, and in general, more delivered capacity variation.
What is needed is an electrochemical cell comprising an electrode assembly having an anode and a cathode that maintain close physical proximity to each other throughout the entire discharge life of the cell.